The present invention relates to an optical modulator for varying intensity, frequency, phase and other information on input light in response to voltages which are applied thereto from the outside.
A semiconductor laser is extensively used as a light source for an optical communication system and others. Methods of varying the light output intensity, phase and frequency of a semiconductor laser at high speed known in the art may generally be classified into two types, i.e., a method which directly varies an injection current adapted to drive a laser and a method which modulates a light beam issuing from a light source by passing it through an optical modulator. The current modulation type method is free from insertion loss due to an optical modulator because it does not rely on an optical modulator. However, the problems with such a method are that during high-speed modulation above several hundreds of megahertz it is difficult to detect a signal beam due to distortions of a modulated wave, which are caused by relaxation oscillation of carriers in the laser, and chirp of lasing wavelength, and that direct modulation above about 4 gigabits per second is almost impractical because the modulation rate is limited by carrier lifetime. Meanwhile, the beam modulation type method which uses an optical modulator can perform high-speed modulation on the order of 10 gigabits per second and allows a minimum of chirp to occur even during high-speed modulation. Nevertheless, where use is made of an ordinary optical modulator, the beam modulation type method brings about significant insertion loss and, therefore, cannot be advantageously applied to communications, especially long-distance communications. In addition, a high on/off ratio is unattainable unless the optical modulator is driven by high voltages.
In relation to the beam modulation type scheme, there has been proposed an optical modulator which, implemented with a multi-layer semiconductor, is small in loss and capable of performing high-speed modulation. An example of such a kind of optical modulators is disclosed in T. H. Wood et al. "High-speed optical modulation with GaAs/GaAlAs quantum wells in a p-i-n diode structure", Appl. Phys. Lett., Vol. 44, No. 1, pp. 16-18, 1984. In this example, an electric field is applied to ultrathin semiconductor layers so that due to the resultant quantum confined startk effect the absorption edge is shifted toward longer wavelengths. An effort is made in this example to achieve a high on/off ratio taking advantage of the shift, as is caused by that of the absorption edge, of a sharp absorption peak due to exciton which exists even at room temperature by virtue of quantum size effects. However, the effect attainable with such a scheme is limited because in an ultrathin layer structure heretofore contemplated the energy value of a conduction band or valence band is spatially constant and, therefore, any change in the band structure caused by an externally derived electric field entails a decrease in the overlap integral of a wave function representative of a probability of existence of electrons in the conduction band and a one representative of a probability of existence of holes in the valence band, while entailing a decrease in the absorption coefficient itself.